Polyethylene terephthalate, commonly abbreviated “PET,” consists of polymerized units of the monomer ethylene terephthalate, with repeating (C10H8O4) units. PET is the most common thermoplastic polymer resin of the polyester family and is used in fibers for clothing and carpets, containers for liquids and foods, and as componentry, among other things. The majority of the world's PET production is for synthetic fibers (in excess of 60%), with bottle production accounting for about 30% of global demand. Polyester makes up about 18% of global polymer production.
While PET is recyclable in many locations, typically PET waste has been landfilled due to the cost of recycling this material into useful materials. It has not been cost effective to recycle PET into useable new materials because it has been cheaper to generate materials from petrochemical materials than from the depolymerization of waste PET which, when coupled with the general availability of landfill space, has disincentivized PET recycling. However, given the increasing cost and environmental impact of petroleum-derived polymers like PET, as well as the decreasing availability of landfill space in many parts of the world, recycling of PET is becoming more desirable. Thus, there is an increasing focus on development of cost effective and value-added methods to recycle PET to generate new products.
Terminology for plastics recycling includes four categories:                Primary (mechanical reprocessing into a product with equivalent properties) is often referred to as “closed-loop” recycling;        Secondary (mechanical reprocessing into products having reduced property requirements than the previous polymer), referred to as “downgrading;”        Tertiary is described as “chemical” or “feedstock” recycling and applies when the polymer is de-polymerized to its chemical constituents to be used to generate new polymers or other useful chemicals; and        Quaternary is energy recovery, energy from waste, such as by burning for fuel to utilize the petrochemical components therein.        
In general terms, tertiary recycling has the advantage of recovering the petrochemical constituents of the polymer, which can then be used to re-manufacture the polymer or to make other chemicals. However, while technically feasible, it has generally been found to be uneconomic without significant government or other subsidies because of the low price of petrochemical feedstock as compared to the plant and process costs incurred to produce monomers from waste polymers. This is not surprising because depolymerization effectively involves reversing the energy-intensive polymerization previously carried out during first order PET manufacturing processes.
One of the useful materials that has been a goal of tertiary PET recycling is the generation of aromatic polyester polyols as raw materials for polyurethane polymers, that is, to substitute for the petrochemical feedstock that would otherwise be needed to obtain these materials. When the aromatic polyester polyether polyol is generated natively—that is, not from the recycling of PET—the aromatic polyester polyol can be made by condensing aromatic diacids, diesters, or anhydrides (e.g., terephthalic acid, dimethyl terephthalate) with glycols such as ethylene glycol, propylene glycol, diethylene glycol, or the like. When PET waste is depolymerized for generation of aromatic polyester polyol via glycolysis, ethylene glycol, diethylene glycol, propylene glycol, or dipropylene glycol are typically used. Ethylene glycol has been reported as the most reactive glycol for PET glycolysis. As would be appreciated, transesterification via glycolysis converts the polymer to a mixture of glycols and low-molecular-weight PET oligomers, and the transesterification products can be modified for use with a variety of chemicals after completion of glycolysis to provide an aromatic polyester polyol that is workable in a polyurethane reaction.
Such aromatic polyester polyol products of the glycolysis of PET can be used in the preparation rigid polyurethane foams (“RPUF”). RPUFs can be used as insulating materials due to their generally low thermal conductivity. Such foams can be used, for example, as outer wall insulation of residential and commercial buildings, shipping containers (e.g., tractor trailers, rail cars, shipping containers etc.) and pipelining, among other things. Unfoamed polyurethane materials can also be used for coatings, adhesives, and sealants.
The aromatic content of aromatic polyester polyols derived from PET are known to contribute to the strength, stiffness, and thermal stability of the polyurethane product. RPUFs generated from aromatic polyester polyols have been shown to exhibit excellent overall performance in insulation applications. Thermal stability of RPUFs depends on the polyol structure, and aromatic polyols can be superior over aliphatic polyols from this point of view. Previously introduced aromatic polyester polyether polyols based on terephthalic acid or phthalic anhydride have a high content of aromatic fragments, for example, about around 20%. The presence of aromatic fragments in the structure of polyols has been shown to enhance many properties of RPUF enabling good mechanical characteristics, high thermal stability, resistance to major chemical solvents, and low flammability. Nonetheless, the prevailing price charged for existing RPUFs generated from prior art aromatic polyester polyols makes this material much less desirable than that of foamed polystyrene or mineral wool products for commercial applications.
To this end, starting materials for both polyols for use in aromatic polyester polyols are typically derived exclusively from petrochemical sources. At least because the recycling of PET into useful articles reduces some use of non-renewable materials, it would seem desirable to use such material as an upstream feedstock for polyurethane manufacture. However, the economics of such manufacture does not support this use case. One might also infer that a price differential might be possible for aromatic polyester polyether polyol derived from recycled PET in that people would be willing to pay more from polyurethane materials derived from such a source, but this is not the case given current methods proposed for generation of these materials. Users are demanding “greener” products that perform equally well to existing, petroleum-derived products, but they are not typically willing to pay a premium for these products. Reduction in the price of RPUFs made from aromatic polyester polyols could become possible by the use of PET-waste derived as raw materials.
In view of the foregoing, it would be desirable to develop methods and materials that could enhance the ability to use waste PET to generate chemical feedstock that can be used to generate high value materials, while still addressing the cost requirements demanded by consumers for such high value materials. Yet further, it would be desirable to develop polyurethane materials that exhibit improved chemical and physical properties. The present invention provides these and other benefits.